Page 188 - Well Logging and Formation Evaluation
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178 Well Logging and Formation Evaluation
0.002
0.0015
0.001
Field (T) 0.0005 Ftot
Fxy
Fax
0
960 980 1000 1020 1040 1060 1080 1100 1120 1140
–0.0005
–0.001
depth
Figure 12.1.2 Field Due to a Monopole: F tot, F xy, F ax
Consider a general case in which a survey well passes a target well
with the closest approach distance of d (see Figure 12.1.8). Let a mono-
pole be present on the target at a height of t above the point of closest
approach.
The point at which the sensor is closest to the pole AX dir will become
90 degrees. Note that this is not necessarily the point at which the wells
themselves are closest together. Also, although one well passes the other,
HS dir remains constant with alonghole depth in the survey well. The short-
est distance between the survey well and pole (x) may be determined from
measuring the width of the F tot curve at half the maximum intensity and
dividing by 2. Hence:
x =D F tot 2. (12.1.20)
Alternatively, the F xy curve may be used, but the formula becomes:
